1. Field of the Invention
The invention relates in general to a method of forming an integrated circuit and more particularly to a method of forming a shallow trench isolation (STI).
2. Description of the Related Art
In the very advanced fabrication technology of integrated circuits to reduce the dimensions of devices and to increase the integration are a leading trend and topic for further development. However, as the dimensions of devices shrink, the isolation structures between devices have to shrink as well. It is thus to cause a problem and difficulty in fabrication. Isolation structure such as a field oxide formed by local oxidation (LOCOS) has been widely used in the conventional fabrication process. Due to the consequently caused characteristics such as a bird's beak. This technique cannot meet the requirement for higher integration. Other structure such as a shallow trench isolation has been used instead of the field oxide layer, especially in sub-half micron fabrication process.
To fabricate a shallow trench isolation, a nitride layer is commonly used as a hard mask layer on a substrate. Using anisotropic etching, a trench is formed in the substrate. An oxide plug is then filled in the trench to form the shallow trench isolation. In the conventional method, it is inevitable that a recess occurs around the edge of the oxide plug to cause a corner effect. In the subsequent process, such as using ion implantation to form a source/drain region of a transistor in the substrate, the implanted charged ion would accumulate in the recess around edge. An abnormal subthreshold current is caused in a channel region of the transistor due to accumulated charges. That is, a kink effect is caused. The corner effect has been further discussed by Geissler, Poth, Lasky, Johnson, and Voldman in the paper "A New Three-Dimensional MOSFET Gate-Induced Drain Leakage Effect in Narrow Deep Submicron Device" published in IEEE IEDM Technical Digest, 1911.
To solve the problem of corner effect, Fazan and Pierre C. disclosed a method for fabricating a shallow trench isolation in U.S. Pat. No. 5,799,383. In this disclosure, after the formation of all oxide plug, an oxide layer is formed to cover the substrate and the oxide plug. The oxide layer is etched back to leave a spacer to cover a sidewall of the oxide plugs. Using wet etching, a pad oxide layer previously formed on the substrate is removed. However, it is known that the step of etching back the oxide layer is performed by a dry plasma etching process. Since the materials of the oxide layer and the pad oxide layer are apparently the same, the selectivity between these two layers for dry etching is so low that there is no effective way to control the etching level. As a consequence, the pad oxide layer is consequently removed while etching back the oxide layer. The substrate is very likely to be exposed under a plasma environment to be damaged by the plasma. Moreover, in the subsequently process such as an ion implantation, the substrate is directly exposed to the high energy implanted ions. The substrate is thus further damaged. Therefore, though this technique disclosed here improve the corner effect the substrate is easily damaged by directly exposed under a plasma or implanted ions.